🚀 go-pugleaf

Why doesn't ReportMouse() work for me?

I have a window, I have an IDCMP.  I would like to recieve MOUSEMOVE
messages only when the left button is down, so I thought I would watch
for MOUSEBUTTON messages and call ReportMouse(window,TRUE) when I get
a class==SELECTDOWN, and ReportMouse(window,FALSE) when I get a
class==SELECTUP.  Well, I am having problems with ReportMouse().

When I call ReportMouse(window,FALSE), nothing happens (I continue to
recieve MOUSEMOVEs), and when I call ReportMouse(window,TRUE), I get a
big ol' guru!  This happens no matter where in my program I call it.

I thought I might achieve the same thing instead by using ModifyIDCMP,
and changing MOUSEMOVE when I get the appropriate MOUSEBUTTONs (though
I don't like this approach as much).  Well, whenever I call
ModifyIDCMP(window, anyflags), I lose MOUSEMOVEs totally.  Even if
MOUSEMOVEs are on and I request that they stay on.  Can anyone tell me
what's wrong?  Is there something conceptually wrong with this?  Or am
I just missing some detail?
More Info:
"window" is a pointer to a struct window as returned from OpenWindow.
My window is a backdrop window, but I have tried it on a normal window too.
I am reading messages off of window->UserPort.
And here's a small piece of code (from my event loop):

if (class & MOUSEBUTTONS) {
        if (code & SELECTDOWN) ModifyIDCMP(window,MOUSEMOVE | IDCMPflags);
        if (code & SELECTUP) ModifyIDCMP(window,IDCMPflags);
-or-
if (class & MOUSEBUTTONS) {
        if (code & SELECTDOWN) ReportMouse(window,TRUE);
        if (code & SELECTUP) ReportMouse(window,FALSE);

(please cc: me a copy of replies, I'll get it quicker than wading
through posts)         --demetri <dp1g@andrew.cmu.edu>